1. Field of the Invention
The present invention relates to a single crystal for a scintillator and a method for manufacturing the single crystal. More particularly, the present invention relates to a to a single crystal for a scintillator for use in single crystal scintillation detectors (scintillators) of radiation such as gamma radiation in the field of radiation medicine, physics, physiology, chemistry, mineralogy, and oil prospecting, e.g., for positron CT (PET) for medical diagnosis, cosmic ray observations, and exploration of underground resources, and also to a method for manufacturing the single crystal.
2. Related Background Art
Scintillators of cerium-activated gadolinium orthosilicate have been used as radiation detectors, e.g., for positron CT, due to their short fluorescence decay time and a high radiation absorption coefficient. The fluorescence output of such scintillators is higher than that of BGO scintillators, but is only about 20% that of NaI (Tl) scintillators, and in this respect a significant improvement thereof is needed.
Scintillators using a single crystal of cerium-activated lutetium orthosilicate represented by Lu2(1−x)Ce2xSiO5 (see specifications of Japanese Patent No. 2,852,944 and U.S. Pat. No. 4,958,080) are generally known. Scintillators using a single crystal of a compound represented by Gd2−(x+y)LnxCeySiO5 (Ln is Lu or one of rare earth elements) are also generally known (see specifications of Japanese Published Examined Patent Application No. 7-78215 and U.S. Pat. No. 5,264,154) are also generally known. These scintillators are known to have an increased density of crystals, and also an increased fluorescence output of single crystals of cerium-activated orthosilicate compounds and a shortened fluorescence decay time.
However, it was found that when a specific single crystal of cerium-activated orthosilicate compound is grown or cooled in an atmosphere comprising oxygen (for example, an atmosphere with an oxygen concentration of 0.2 vol. % or more) or when the single crystal is grown in an atmosphere with a low concentration of oxygen, if the single crystal is thereafter subjected to high-temperature heat treatment in an atmosphere comprising oxygen, the heat treatment causes coloration of the crystal and decrease in fluorescence output due to fluorescence absorption or the like (for example, see specification of Japanese Patent No. 2,701,577). Further, because single crystals of cerium-activated orthosilicate compounds have a high melting point, they are generally grown by a Czochralski method based on high-frequency heating in an Ir crucible. However, if an Ir crucible is heated to a high temperature in an atmosphere comprising oxygen, the crucible is evaporated and stable crystal growth is difficult to implement.
A method for heat treating a single crystal of cerium-activated gadolinium orthosilicate at a high temperature (at a temperature 50 to 550° C. lower than the melting point of the single crystal) in an atmosphere with a low concentration of oxygen is disclosed in Japanese Patent No. 2,701,577 as a method improving scintillation characteristics such as fluorescence output and energy resolution. According to the aforementioned document, the scintillation characteristics are improved due to the reduction of tetravalent Ce ions, which hinder the scintillation emission, into trivalent ions.
Further, Published Japanese translation of a PCT application No. 2001-524163 describes a scintillation material based on a silicate crystal comprising lutetium (Lu) and cerium (Ce), this material having an oxygen vacancy □ and the chemical composition of the material being represented by general formula (8) below.Lu1−yMeyA1−xCexSiO5−z□z  (8)In formula (8), A stands for Lu and at least one element selected from a group consisting of Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb; and Me stands for at least one element selected from a group consisting of H, Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Ti, Pb, Bi, U, Th.
Published Japanese translation of a PCT application No. 2001-524163 describes more than 50 elements from H to Th as Me that substitute Lu. These substitution elements are described to be effective in preventing the crystal from cracking when a scintillation element is cut and manufactured and for producing a waveguide characteristic in a waveguide element. Further, it is also described that if ions with a degree of oxidation of +4, +5, +6 (for example, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W, Th) are present in the starting formulation, or if the necessary amount thereof is added to the scintillation material, these ions effectively prevent the crystal from cracking and also inhibit the formation of vacancies in the oxygen sublattice.
A method of heating a single crystal of tungsten oxide in an atmosphere comprising oxygen at a temperature within a range from the melting point of the crystal (excluding the melting point) to the temperature that is by 200° C. lower than the melting point or to a lower temperature is disclosed in Japanese Published Examined Patent Application No. 64-6160 as a heat treatment method that increases the fluorescence output of an oxide scintillator. Japanese Published Examined Patent Application No. 64-6160 describes that oxygen vacancies are easily generated in a single crystal of tungsten oxide and that the fluorescence output is increased by heating the crystal at a temperature close to the melting point in an atmosphere comprising oxygen to eliminate the oxygen vacancies.
Japanese Patent Application Laid-open No. 2003-300795 discloses that a single crystal of Gd(2−x)CexMeySiO5 (x is 0.003 to 0.05, y is 0.00005 to 0.005, Me is an element selected from a group consisting of Mg, Ta, and Zr, or a mixture thereof) with no coloration and high transparency is obtained because the element represented by Me inhibits the valence change of Ce ions from 3 to 4.
Further, Japanese Patent Application Laid-open No. 9-142994 describes that when a single crystal of a rare earth silicate for use in a scintillator or the like is grown, the number of voids in the crystal is decreased by reducing the concentration of Al impurity in the raw material of rare earth oxide or the like to 0.4 ppm and below. Japanese Patent Application Laid-open No. 2004-59382 describes that a fluorescence decay time is shortened, while the scintillator characteristic is maintained, by incorporating Al in an amount of more than 0.4 ppm to 500 ppm or less to a single crystal of a rare earth silicate.